It's easy to find frames that are not specifically step through frames both new and used. However step through frames are apparently a very niche market.
If you want new, I can only suggest you find the style of step through frame you want on a bike already made and then find out if the manufacturer offers that as a frame only option. You might find a new frame offered by one of the sellers on Ali Express. For used frames, eBay, craigslist, facebook marketplace and others. And for used, an entire bike at the right price might be worth stripping and re-building.
But again, what's your style? eBay has plenty of used step through frames. Many for way more than I'd ever pay for a frame. Few if any I saw were aluminum.